1 Oct 2012

Potten End Newsletter Article 5

This article originally appeared in the Potten End Newsletter, October 2012.

Potten End Weather

“Autumn is a second spring when every leaf is a flower.”
― Albert Camus
Autumn is upon us, it’s getting colder and the nights are drawing in. Long range ‘White Christmas’ forecasts are being published but at the moment these are more for entertainment value than serious contingency planning. If you’re interested, the chances of a white Christmas are currently 20% south of Yorkshire and Lancashire and 25% north of there.

The meteorological seasons are different to the commonly seen astronomical seasons which are based on the equinoxes and solstices. In 1780 the Societas Meteorologica Palatina, an early international organisation for meteorology, defined seasons as groupings of three whole months. As a result in the northern hemisphere autumn starts on 1st September, winter starts on 1st December, spring starts on 1st March and summer starts on 1st June.

Wind Chill

With the increasingly cold weather, we’ll start to hear about wind chill in the forecasts. More recently this has been referred to as the ‘Feels Like‘ temperature. It’s the apparent temperature felt by people (as well as other warm blooded creatures) during cold and windy conditions. It’s not just a rough figure guessed at by forecasters. There is a sophisticated formula used to calculate the ‘Feels Like’ temperature that takes into account  things like modern heat transfer theory, skin tissue resistance and, of course, wind speed at around 5 feet which is the average height of a human face. We use the formula to provide the ‘Feels Like’ temperature on our web page (www.pottenend.org). It’s only calculated for temperatures below 10°C.

Jet Stream

This past summer, the Jet Stream has played a huge part in making our weather foul or fabulous. Existence of the Jet Stream was hinted at after the 1883 eruption of the Krakatoa volcano. The plume of ash from the volcano was observed over several years and became known as the “equatorial smoke stream”. In the 1920s, a Japanese meteorologist observed the jet stream using weather balloons launched near Mount Fuji. It wasn’t until 1939 that the term Jet Stream was coined by German meteorologist Heinrich Seilkopf. The original German term was Strahlstr√∂mung (or Strahlstrom in modern German). The real understanding of the nature of Jet Streams came during World War II when transatlantic flyers noticed tail winds in excess of 100mph.

There are two jet streams per hemisphere. The subtropical and the polar. Occasionally they merge in to one. It is the polar jet stream that broadly defines our weather and depending on whether it moves further north or further south, the weather is warmer or colder. In general, if it moves north, then the UK has better, warmer weather. This year, and in 2007, the polar jet stream stayed mostly south for the summer which resulted in severe flooding.

Outlook

We’re expecting nothing unusual for October’s weather. A low pressure area will dominate, bringing rain and temperatures of around 14-17°C which is average or slightly above average. We may enjoy the odd bright spell, but not for long.

In the night sky we can expect to see a great show of Orionid meteors from October 20th to the 24th in the south east. There should be about 20 pale yellow and blue meteors an hour. The Orionid meteors are produced by dust from the tail of Halley’s comet.